Electrode compositions

ABSTRACT

A display device including an anode, a cathode, and a region of an organic electroluminescent material located between the anode and the cathode, wherein the organic electroluminescent material is a blue-light emitter, and the cathode includes a first layer and a second layer located between the first layer and the organic electroluminescent material, the first layer containing aluminum and the second layer containing at least one of sodium fluoride and potassium fluoride.

This invention relates to compositions of electrodes for light-emissivedevices, especially for devices that emit light by means oflight-emissive organic materials.

An emerging class of display devices uses organic materials for lightemission. Light-emissive organic materials are described in PCTWO90/13148 and U.S. Pat. No. 4,539,507, the contents of both of whichare incorporated herein by reference. The basic structure of thesedevices is a light-emissive organic layer, for instance a film of apoly(p-phenylenevinylene (“PPV”), sandwiched between two electrodes. Oneof the electrodes (the cathode) injects negative charge carriers(electrons) and the other electrode (the anode) injects positive chargecarriers (holes). The electrons and holes combine in the organic layer,generating photons. In PCT WO90/13148 the organic light-emissivematerial is a polymer. In U.S. Pat. No. 4,539,507 the organiclight-emissive material is of the class known as small moleculematerials, such as (8-hydroxyquinolino)aluminium (“Alq3”). In apractical device, one of the electrodes is typically transparent, toallow the photons to escape the device.

FIG. 1 illustrates the cross-sectional structure of a typical organiclight-emissive device (“OLED”). The OLED is typically fabricated on aglass or plastic substrate 1 coated with a transparent first electrode 2such as indium-tin-oxide (“ITO”). Such coated substrates arecommercially available. This ITO-coated substrate is covered with atleast a layer of a thin film of an electroluminescent organic material 3and a final layer forming a second electrode 4, which is typically ametal or alloy. Other layers can be added to the device, for example toimprove charge transport between the electrodes and theelectroluminescent material. When a voltage is applied between theelectrodes from a power supply 5 one of the electrodes acts as a cathodeand the other as an anode.

The nature of the electrodes has a strong influence on the efficiency ofthe device. For the cathode electrode a number of materials have beenproposed, with materials having a low work-function being generallypreferred.

-   -   U.S. Pat. No. 4,539,507 suggests cathodes of metals such as        indium, silver, tin, lead, magnesium and aluminium.    -   WO 00/48257 describes a trilayer cathode having a layer of        aluminium, a layer of calcium and a layer of lithium or        magnesium fluoride.    -   EP 0 822 603 A proposes a bilayer cathode which includes a thin        fluoride layer and a thick conductive layer. The fluoride can be        selected from the group of alkaline fluorides and alkaline earth        fluorides. The conductive layer can be selected from the group        of elemental metals, metal alloys and conductive materials. For        the fluoride layer thicknesses in the range 0.3 to 5.0 nm are        taught.

According to one aspect of the present invention there is provided adisplay device comprising: an anode; a cathode; and a region of anorganic electroluminescent material located between the anode and thecathode; wherein: the organic electroluminescent material is ablue-light emitter; and the cathode comprises a first layer and a secondlayer located between the first layer and the organic electroluminescentmaterial, the first layer comprising aluminium and the second layercomprising at least one of sodium fluoride and potassium fluoride.

According to a second aspect of the present invention there is provideda method for forming a display device, comprising: forming a structurecomprising an anode and a region of an organic electroluminescentmaterial; depositing in contact with the organic electroluminescentmaterial a cathode comprising a first layer and a second layer locatedbetween the first layer and the organic electroluminescent material, thefirst layer comprising aluminium and the second layer comprising atleast one of sodium fluoride and potassium fluoride.

Preferably the first layer consists essentially of aluminium.

Preferably the second layer consists essentially of sodium fluoride andpotassium fluoride. The second layer may comprise only one of sodiumfluoride or potassium fluoride.

The thickness of the first layer is suitably in the range from 200 to700 nm, preferably in the range from 300 to 600 nm. The thickness of thesecond layer is suitably in the range from 2 to 6 nm, preferably in therange from 3 to 5 nm.

The organic electroluminescent material is preferably a polymer oroligomer comprising fluorene units, and is most preferably apolyfluorene. The organic electroluminescent material could usefully bea blue emitting copolymer of one or more fluorenes and one or moretriarylamines. The fluorene units preferably contribute to lightemission from the material.

The device may suitably have its peak intensity of emission at awavelength in the region 400 to 500 nm. The device may suitably emitwith a colour having 1931 CIE coordinates in the range 0.1≦x≦0.2,0.00≦y≦0.1, preferably in the range 0.12≦x≦0.18, 0.02≦y≦0.8 and mostpreferably around x=0.15, y=0.05.

The device may constitute a blue pixel of an RGB (red, green, blue)display.

The device may comprise a first power supply coupling on the anode and asecond power supply coupling on the first layer of the cathode.

The second layer is preferably in contact with the organicelectroluminescent material. The first layer is preferably in contactwith the second layer. The first layer is preferably separated by thesecond layer from the organic electroluminescent material, so the firstlayer is preferably not in contact with the organic electroluminescentmaterial.

The first layer is deposited by evaporation on to the organicelectroluminescent material. The deposition rate is preferably between 1and 5 Å/s, preferably around 2 Å/s.

Some or all of the first layer is advantageously deposited byevaporation. Preferably the evaporation rate is less than 1 Å/s .Preferably the material from which the first layer is to be deposited isoffgassed prior to evaporation, for example by being held at an elevatedtemperature below the evaporation temperature. The elevated temperaturemay conveniently be above 500° C. and the material may be held at thattemperature or above for 5 minutes or more.

Some or all of the second layer is advantageously deposited byevaporation. Preferably the evaporation rate is less than 1 Å/s . Oneadvantageous route is to deposit part of the first layer, to a thicknessof 100 nm or more, by evaporation and/or at a deposition rate of lessthan 1 Å/s. That part of the first layer suitably contacts the secondlayer. Conveniently a subsequent first portion of the second layer canbe deposited (e.g. by evaporation) at a rate greater than 5 Å/s.Preferably the material from which the second layer is to be depositedis offgassed prior to evaporation, for example by being held at anelevated temperature below the evaporation temperature. The elevatedtemperature may conveniently be above 500° C. and the material may beheld at that temperature or above for 5 minutes or more.

The organic electroluminescent material is suitably a polymer material,preferably semiconductive polymer material and preferably a conjugated(either fully or partially) polymer material. Alternatively, theelectroluminescent material could be a non-polymeric organic material,such as a small molecule material, an oligomer material or a monomermaterial. The organic electroluminescent material may comprise one, twoor more electroluminescent components, for instance as a mixture or acopolymer. The polymer could be a copolymer including fluorene units.

The device may suitably include one or more additional layers. Oneexample of such an additional layer is a charge transport layer, whichcould be located between an electrode layer and the light-emissivelayer. The or each charge transport layer may suitably comprise one ormore polymers such as polystyrene sulphonic acid doped polyethylenedioxythiophene (“PEDOT-PSS”),poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoicacid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (“BFA”),polyaniline and PPV.

The anode electrode may suitably have a work function of greater than4.3 eV. That electrode may comprise a metallic oxide such as indium-tinoxide (“ITO”) or tin oxide (“TO”).

At least one of the electrodes is suitably light transmissive, andpreferably transparent, suitably to light emitted from thelight-emissive regions.

The blue light emitting devices of the present invention areparticularly suited to use as a white light source when combined with aphosphor-containing covering. An example of such a white light source isdisclosed in WO00/33390. White light sources have application in a widerange of residential, commercial and industrial settings.

The present invention is also directed to a white light emitting devicecomprising:

-   an organic light emitting device comprising an anode, a cathode and    a region of an organic electroluminescent material located between    the anode and the cathode; wherein the organic electroluminescent    material is a blue-light emitter; and-   the cathode comprises a first layer and a second layer located    between the first layer and the organic electroluminescent material,    the first layer comprising aluminium: and the second layer    comprising at least one of sodium fluoride and potassium fluoride,    the white light emitting device further comprising a    phosphor-containing covering at least partially covering the organic    light emitting device, the phosphor-containing covering suitable for    partially absorbing light emitted by the organic electroluminescent    material and emitting light at longer wavelengths such that the    overall emission from the device is white. Preferably the    phosphor-containing covering comprises green-emitting phosphors and    red-emitting phosphors. The green emitting phosphor preferably has    an emission peak at 530-555 nm. The red emitting phosphor preferably    has an emission peak at 610-620 nm. For the purposes of the present    invention white light is considered to be light having 1931 CIE    coordinates of 0.33, 0.41 and/or a colour temperature of    3000-4100° K. The phosphor-containing covering is preferably    situated on the viewing side of the light emitting device and    preferably covers at least 50% of the viewing side of the device.

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 shows the basic structure of an OLED;

FIG. 2 illustrates an OLED having a bilayer cathode;

FIG. 3 is a plot of luminance and efficiency against applied voltage fordevices having cathodes of various materials; and

FIG. 4 is a plot of luminance against time for devices having cathodesof various materials.

FIG. 2 shows an organic light-emissive device having a bilayer cathode.The device comprises a transparent glass or plastics substrate 11. Overthe substrate is a transparent anode electrode 12 formed of ITO. Overthe anode is a layer 13 of organic light-emissive material 13 and overthat is a cathode layer 14. The cathode layer 14 comprises two layers16, 17. Layer 17 is located between layer 16 and the light-emissivelayer 13 and separates layer 16 from layer 13. A power supply 15 isconnected between the anode 12 and layer 16 of the cathode 14. The powersupply is arranged to apply a voltage between the electrodes so as tomake cathode 14 electrically negative with respect to anode 12.

In the cathode, layer 16 is a metal layer. It is formed of aluminium.The thickness of layer 16 is around 100 to 1000 nm, preferably around200 to 700 nm. Layer 17 is a fluoride layer. It is formed of sodiumfluoride or potassium fluoride. The thickness of layer 17 is in therange from 2 to 6 nm, preferably from 3 to 5 nm, and most preferablyaround 4 nm.

The substrate 11 and the anode electrode 12 may be a pre-preparedcommercially available ITO-coated glass sheet. To form the device thelight-emissive layer 13 is deposited over the ITO layer. Thelight-emissive layer can conveniently be deposited from solution, forexample by spin coating. Then the cathode is formed over thelight-emissive layer.

The cathode is preferably formed by evaporation of the fluoride layer 17followed by evaporation of the metal layer 16. It has been found to beadvantageous to evaporate at least the fluoride layer at a very lowrate: preferably less than 1 Å/s, although somewhat higher rates couldbe used. For best results the first part of the metal layer (suitablythe first 100 nm or so of the metal layer) is also deposited at such alow rate. For best results, before the material of each cathode layer isdeposited it is outgassed by being held at an elevated temperature belowits evaporation point—conveniently around 650 to 670° C.—for around 5 to10 minutes.

Devices having cathodes in which layer 16 is formed of aluminium andlayer 17 is formed of sodium fluoride or potassium fluoride have beenfound to have markedly better performance than devices having othercompositions of cathode, even ones in which the fluoride layer is formedof lithium fluoride. This effect has been found to be especiallypronounced when the light-emissive material emits in the blue region ofthe spectrum. One example of a blue light emitter: a copolymercomprising 10% TFB, 10% PFB, 80% F8, is detailed below with reference todevices A to C. Other examples of blue light emitters are given in D YKim et al., Progress in Polymer Science 25 (2000) 1089-1139.

Devices were manufactured having cathode structures as shown in columns2 and 3 of the following table: Layer Layer 17 Luminance/ Efficiency/Luminance/ Device 16 (thickness) voltage plot voltage plot time plot AAl NaF (4 nm) 20 23 26 B Al  KF (2 nm) 21 24 27 C Al  LiF (3 nm) 22 25 —

The light-emissive layer for each of these devices comprised a copolymerof 10% TFB (i.e. bis(1,4-phenylene)-4-sec-butylphenylamine), 10% PFB(i.e. 1,4-phenylene-((4-butylphenyl)imino)-1,4-phenylene((4-butylphenyl) imino)-1,4-phenylene)) and 80% F8(i.e. 9,9-dioctylfluorene). This material is discussed in more detail inWO0/55927.

FIG. 3 plots the luminance and efficiency of the devices against appliedvoltage (see columns 4 and 5 of the above table). The NaF and KF devices(A and B) show markedly better luminance and efficiency than the LiFdevice (C).

FIG. 4 plots the luminance of devices A and B over time. Both devicesshow acceptable lifetimes of around 2000 hours above 50 cd/m². Deviceslike device C in which the layer 16 is formed of aluminium and the layer17 is formed of lithium fluoride have been found to give only very shortlifetimes in devices of this type.

In comparison to the cathodes described in WO 00/48257 devices A and Bhave the advantage that they do not include metallic calcium, which ishighly reactive and has been found in some circumstances to lead todegradation. Also, it is simpler to manufacture a bilayer cathode than atrilayer cathode. Experiments have shown devices having NaF/Al and KF/Alcathodes to be significantly more efficient than devices havingLiF/Ca/Al cathodes.

The light-emissive materials discussed above are preferably organicpolymer, small molecule or oligomer materials. Suitable materialsinclude conjugated fluorenes, amines and copolymers thereof.

The cathode material described above can be used in a common cathodedevice configuration, in which two or more pixels (normally havingdifferent emission colours) share a common cathode but have differentanodes.

A layer of a charge transport material may be present between the anodeand the light-emissive material. The charge transport material could bePEDOT-PSS.

The present invention may include any feature or combination of featuresdisclosed herein either implicitly or explicitly or any generalisationthereof, irrespective of whether it relates to the presently claimedinvention. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

1. A display device comprising: an anode; a cathode; and a region of anorganic electroluminescent material located between the anode and thecathode; wherein; the organic electroluminescent material is ablue-light emitter; and the cathode comprises a first layer and a secondlayer located between the first layer and the organic electroluminescentmaterial, the first layer comprising aluminum and the second layercomprising at least one of sodium fluoride and potassium fluoride.
 2. Adisplay device as claimed in claim 1, wherein the first layer consistsessentially of aluminum.
 3. A display device as claimed in claim 1,wherein the second layer consists essentially of sodium fluoride orpotassium fluoride.
 4. A display device as claimed in claim 1, whereinthe second layer comprises sodium fluoride.
 5. A display device asclaimed in claim 1, wherein the second layer comprises potassiumfluoride.
 6. A display device as claimed in claim 1, wherein thethickness of the first layer is in the range from 200 nm to 700 nm.
 7. Adisplay device as claimed in claim 1, wherein the thickness of thesecond layer is in the range from 2 nm to 6 nm.
 8. A display device asclaimed in claim 1, wherein the organic electroluminescent material is apolyfluorene.
 9. A display device as claimed in claim 9, wherein theorganic electroluminescent material is copolymer of at least onefluorene and at least one triarylamine.
 10. A display device as claimedin claim 1, comprising a first power supply coupling on the anode and asecond power supply coupling on the first layer of the cathode.
 11. Adisplay device as claimed in claim 1, wherein the second layer is incontact with the organic electroluminescent material.
 12. A displaydevice as claimed in claim 1 prepared by a process comprising the stepof depositing the first layer on to the organic electroluminescentmaterial at a rate less than 1 A/s by evaporation.
 13. A method forforming a display device, comprising: forming a structure comprising ananode and a region of an organic electroluminescent material; depositingin contact with the organic electroluminescent material a cathodecomprising a first layer, and a second layer located between the firstlayer and the organic electroluminescent material, the first layercomprising aluminum and the second layer comprising at least one ofsodium fluoride and potassium fluoride.
 14. A method for forming adisplay device as claimed in claim 13, comprising depositing the secondlayer by evaporation.
 15. A method for forming a display device asclaimed in claim 14, comprising depositing the second layer at a rateless than 1 A/s.
 16. A method for forming a display device as claimed inclaim 14, comprising the step of offgassing material from which thesecond layer is to be deposited prior to depositing the first layer. 17.A method for forming a display device as claimed in claim 13, comprisingdepositing the first layer by evaporation.
 18. A method for forming adisplay device as claimed in claim 17, comprising depositing a firstpart of the first layer to be deposited at a rate less than 1 A/s.
 19. Amethod for forming a display device as claimed in claim 18, wherein thethickness of the first part is at least 100 nm.
 20. A method for forminga display device as claimed in claim 18 comprising depositing asubsequent portion of the first layer at a rate greater than 5 A/s. 21.A method for forming a display device as claimed in claim 17, comprisingthe step of offgassing material from which the first layer is to bedeposited prior to depositing the first layer.
 22. A method for forminga display device as claimed in claim 13, wherein the organicelectroluminescent material is a blue-light emitter.
 23. A method forforming a display device as claimed in claim 13, wherein the organicelectroluminescent material is a copolymer of at least one fluorene andat least one triarylamine.
 24. A white light emitting device comprising:an organic light emitting device comprising: an anode; a cathode; and aregion of an organic electroluminescent material located between theanode and the cathode; wherein; the organic electroluminescent materialis a blue-light emitter; and the cathode comprises a first layer and asecond layer located between the first layer and the organicelectroluminescent material, the first layer comprising aluminum and thesecond layer comprising at least one of sodium fluoride and potassiumfluoride, the white light emitting device further comprising aphosphor-containing covering at least partially covering the organiclight emitting device, the phosphor-containing covering being suitablefor partially absorbing light emitted by the organic electroluminescentmaterial and emitting said light at longer wavelengths such that theoverall emission from the device is white.
 25. A white light emittingdevice according to claim 24, wherein the phosphor-containing coveringcomprises green-emitting phosphors and red-emitting phosphors.